Voltage Controlled Oscillators (VCOs) are key components used in Phase-Locked Loops (PLLs). The gain of a VCO, it's change in frequency for a change in control voltage, is a factor in the PLL's open loop gain, and therefore can have an effect on the PLL's overall stability. In FIG. 1 there is shown a block diagram of a prior art PLL, with its open loop gain determined by the following formula:       Open    ⁢                   ⁢    loop    ⁢                   ⁢    gain    =                    K        pd            *              K        cp            *              K        LF            *              K        VCO              M  
Due to variations in integrated circuit processing, the gain of a VCO can vary greatly. These processing variations when coupled with the variations introduced from the processing of the loop filter and the programming variations of the loop filter's frequency divider can cause a stable loop to become marginally stable or unstable. Typical VCO gain variations can reach as much as +/−50% due to these variations.
In FIG. 2, there is shown a prior art VCO 200 whose gain varies with the integrated circuit processing, mostly with the strength of the n-channel processing. The NOR logic gate latches 204-210 are composed of pseudo-NMOS logic to control their threshold levels. The gain of the VCO 200 is predominately affected by the n-channel transistor M1 that is used to convert the control voltage to a control current, acting as a Voltage-to-Current (V/I converter) 202, and the speed of the pseudo-NMOS NOR logic gates 204-210. The gain response for VCO 200 is shown in FIG. 3 for the case of nominal (nom), strong, and weak processing scenarios.
In order to combat process variations in VCO's some prior art designs have relied on trimming techniques during the testing of the integrated circuit manufacturing. This solution however increases the test time and thus causes the manufacturing costs to increase. Another prior art technique used for minimizing the process variation problem is to add positive feedback to the VCO design; this however can present stability issues to the VCO design. Given the above discussion, a need exists in the art for a VCO whose gain remains substantially constant over process variations.